In many blasting applications it is desirable to have a charging method providing explosive of reduced and variable bulk strength. In driving tunnels or galleries careful blasting of the contour holes will give a substantially undamaged rock face with strongly reduced needs for subsequent repair and support work such as bolting, gunniting, concrete reinforcement etc. and the final profile will be true to the design size. Similar considerations may arise in underground mining and stopig or for the purpose of limiting production of fines to meet certain after-processing constraints.
Although numerous small and closely spaced bore-holes can be used to produce smooth fracture planes, the method is limited by practical and economical reasons and conventionally careful blasting has been carried out by partial charging of oversized boreholes with small-diameter cartridges or tubes. Another approach is the arrangement of spatially separated and individually ignited deck charges at regular intervals in the borehole. The methods are expensive both in labour and equipment. Frequent problems are inconsistency in charging and uncontrolled coupling between explosive and rock. Detonation failures have also been experienced for certain explosives, supposedly due to precompression from forerunning shock waves in the free gas channel. Introduction of shells or spacers concentric with the charge have improved positioning but added to cost and complicated charging procedure.
To meet the general trend towards wider boreholes and bulk charging of explosives also in connection with careful blasting, bulk explosives of strongly reduced energy concentration have been developed, such as ANFO mixed with porous lightweight material. The complete fill out of large drill holes with explosive places severe demands for energy reduction and the explosive often approaches its detonation limit. Although the positioning problems mentioned in connection with the packaged products are avoided with bulk explosives, the coupling to the rock surface is stronger and the blast result will be markedly dependent on any inhomogenity present in the explosive. These problems are pronounced by the pulverulent nature of the explosives used. The lightweight materials usually employed for energy reduction are not easily mixed with the heavier standard components of the explosive. Precautions taken at manufacture to secure thorough mixing are not sufficient since the components tend to separate during transport and charging operation. The U.S. Pat. No. 4,995,925 describes an improved composition of this kind in which the segregation problems are controlled per se. The general problems with bore-holes filled up with reduced explosives are not solved, however, nor is the need for use of a single explosive composition for multiple strength requirement met by such explosives.
The U.S. Pat. No. 5,105,743 describes a method by which a standard blowable explosive is used to partially fill up a bore-hole. The method is limited to granular and blowable explosives and is of limited use in for example wet environments or other situations when pumpable explosives are needed. The method requires different tools for different bore-hole diameters and tend to give uneven amounts along the hole.
Unlike granular explosives, coherent and pumpable explosives of reduced diameter are susceptible to detonation propagation problems. Under proper detonation they tend to sustain a high detonation velocity, both unconfined and fully confined, which is not always consistent with cautious blasting requirements.